An insulated gate switching device such as a MOSFET (metal oxide semiconductor field effect transistor) or an IGBT (Insulated Gate Bipolar Transistor) has been known. While the insulated gate switching device is turned off, an electric field is applied to a gate insulating film. If the electric field applied to the gate insulating film exceeds an insulation resistance of the gate insulating film, a current flows penetrating the gate insulating film, which is problematic. Accordingly, a technique to relax an electric field applied to the gate insulating film is needed.
Japanese Patent Application Publication No. 2013-191734 A discloses an insulated gate switching device formed in a SiC semiconductor substrate. This insulated gate switching device has a trench-type gate electrode. An electric field relaxation region of p-type is formed under the trench. When the insulated gate switching device is turned off, a depletion layer spreads from the electric field relaxation region to a drift region (an n-type region) on the periphery thereof. With the depletion layer spreading from the electric field relaxation region to the drift region, the electric field applied to the gate insulating film is relaxed.